1. Field of the invention.
The invention is a mechanism for rotationally articulating a pointed device relative to a compensating inertia.
2. Description of Prior Art
Vibrations in large flexible space vehicles cause pointing disturbances for precision instruments attached to the base vehicle. Pointing systems which torque the pointed devices relative to the flexible base vehicle can cause base vehicle structural instabilities and disturb other pointed devices. Momentum compensation schemes have been used to torque pointed devices relative to rigid compensating inertias to minimize base vehicle disturbances, but these momentum compensating devices have been excessively massive or introduce unacceptable pointing disturbances due to mechanism nonlinearities. To minimize the mass of compensating inertias, they are generally rotated more rapidly in the opposite direction than the devices they compensate. The ratio of the compensating inertia angular rate to the pointed device angular rate must be kept constant to assure accurate momentum compensation. Momentum compensation methods include motor driven pointed devices with motor driven compensating inertias, or one motor with mechanical speed reduction methods such as belts and pullies or gear trains between the pointed device and compensating inertia. Momentum compensation methods that require measurement of the pointed device and compensating inertia rotational rates do not have adequate sensors or controllers to provide the fixed rotational rate ratio required for precision pointing. Mechanically slaved pointed devices and compensating inertias have fixed rotational rate ratios but excessive backlash and other nonlinearities result in discontinuities in the fixed rotational rate ratio at rotational rate polarity changes making them unsuitable for precision pointing applications. The "harmonic drive" speed reduction device is ideally suited as a momentum compensating mechanism because it has a large single step rotational speed reduction with negligible backlash. The problem to date has been defining a momentum compensation configuration that is compatible with the idiosyncracies of the harmonic drive mechanism and precision pointing requirements. Actuator designs that avoid torsional windup in the torque transmission path torque the compensating inertia relative to the pointed device and use the harmonic drive to assure a fixed rotational speed ratio between the pointed device and the compensating inertia relative to the base vehicle. The problems with the existing designs are:
1. The harmonic drive ring gear is a rigid cylinder attached to the base vehicle with teeth on the internal cylindrical surface which are forced in contact with the external teeth of a flexible cylindrical flexspline by a rigid eliptical wave generator rotating on the cylindrical axis common to wave generator, ring gear and flexspline. Ring gear eccentricities and ring gear-flexspline tooth mesh errors result in translation of the instantaneous wave generator rotational axis in the plane orthogonal to the fixed ring gear cylindrical axis. Current methods used to accomodate the translation of the wave generator rotational axis are: 1. to support the rigid wave generator in a cantilevered fashion which permits rotational axis translation, or 2. support the wave generator in a loose coupling that permits the wave generator to translate relative to a rigid rotational axis. Method 1 is torsionally stiff with no bachklash, but does not provide any means to preload rotational element support bearings, and method 2 introduces rotational backlash. PA0 2. The ball bearings which support the pointed device, and other rotating elements do not have a rotational axis preload mechanism which is the means to effect a controlled amount of bearing ball shear between the outer and inner ball bearing races. The bearing preload is the means to provide a rotationally true axis and prevent wobble from mass centers off the rotational axis. PA0 3. Periodic base vehicle angular rate periodically passes through zero. Existing actuator designs have all rotational elements angular rates proportional to base vehicle angular rate so that all rotation element bearing races of the pointed devices simultaneously pass through zero angular rate relative to the base vehicle. At this instant, extremely nonlinear stiction friction torques act on the pointed device which must be precisly offset by the pointing system to effect precision pointing. Current sensors, actuators and control systems do not have adequate resolution or response time to precisly offset the nonlinear torques and pointing error results. PA0 1. provide a harmonic drive wave generator shaft coupling means that has no backlash, high torsional stiffness, and low resistance to translational forces orthogonal to the rotational axis. PA0 2. provide a means to provide adjustable axial preload forces to all pointed device rotational support ball bearings. PA0 3. provide a means to bias the angular rate across pointed device support bearings so that the angular rate of each bearing race is never the same avoiding the nonlinear stiction friction torque disturbances.
Accordingly it is the object of this invention to provide the following improvements in the momentum compensated rotary actuator with harmonic drive: